Viscosity modifiers including star polymers are known in the field of lubricants for providing viscosity index performance, low temperature performance as described by Brookfield viscosity and higher temperature performance as indicated by kinematic viscosity performance at 40° C. and 100° C. The viscosity modifiers performance has been observed in a wide variety of mechanical devices including hydraulic systems, driveline systems and internal combustion engines. The star polymers are described in detail in a number of patent applications.
WO 04/087850 and WO 07/025,837 disclose lubricating composition containing block copolymers prepared from RAFT (Reversible Addition Fragmentation Transfer) or ATRP (Atom Transfer Radical Polymerisation) polymerisation processes.
International Applications WO 06/047393, WO 06/047398, WO 07/127,615 (U.S. 60/745,422), WO 07/127,660 (U.S. 60/745,420), WO 07/127,663 (U.S. 60/745,417), and WO 07/127,661 (U.S. 60/745,425) all disclose RAFT polymers for lubricants. The RAFT polymers provide thickening to a lubricant.
International Application WO 96/23012 discloses star-branched polymers prepared from acrylic or methacrylic monomers. The polymers have a core or nucleus derived from acrylate or methacrylate esters of polyols. Further the polymers have molecular weights and other physical characteristics that make them useful for lubricating oil compositions. The star-branched polymers disclosed are prepared by anionic polymerisation techniques.
The star polymers of EP 979 834 require from 5 to 10 weight percent of a C16-30 alkyl(meth)acrylate and from 5 to 15 weight percent of butyl methacrylate. A viscosity index improver with a C16-30 alkyl(meth)acrylate monomer present at 5 weight percent or more has reduced low temperature viscosity performance because the polymer has a waxy texture.
U.S. Pat. No. 5,070,131 discloses gear oil compositions having improved shear stability index essentially consisting of gear oil, a viscosity index improver comprising a hydrogenated star polymer comprising at least four arms, the arms comprising, before hydrogenation, polymerized conjugated diolefin monomer units and the arms having a number average molecular weight within the range of 3,000 to 15,000.
The star polymers described in the state of the art are predominately random or tapered block copolymers.